To reach climate targets, unprecedented levels of energy efficiency improvements are required. To prioritise investments, it is necessary to know the energy saving potential associated with each action. Understanding the potential of technical improvements, requires knowledge on the highest technically achievable efficiency of a technology – the technical efficiency limit. When focusing on technical efficiency improvements, two distinct types of technical systems are recognised: conversion devices and passive systems. Previous research has analysed the technical efficiency limits of passive systems, in this study, the technical efficiency limits of major conversion devices are quantified using physical models. The resulting limits are used to calculate stochastically the energy saving potential of each device and design parameter for the United Kingdom. The UK's final energy demand could be reduced by 25% if conversion devices were operated at their technical limit and two thirds of these savings are in transport. The analysis suggests that a) improvements in conversion efficiencies are insufficient to reach energy reduction targets, except in transport and b) that for most technologies it is more important to focus on converging towards the efficiency level of the best available technologies rather than on research pushing the boundaries of conversion efficiency.